This invention relates to the processing of seismic data to aid in predicting pore pressure, and, more particularly, to the processing of three-dimensional (3-D) seismic data to aid in predicting pore pressure of a subsurface underneath a region of a floor of an ocean, from a surface of the ocean above the floor.
A pore pressure gradient is a measure of the change in the pressure exerted on fluids, in the pores of buried rocks, as a function of depth. Pore pressure gradients vary as a function of depth of burial, depositional history, compaction, mineralogy, and other environmental conditions. A normally (i.e., hydrostatically) compacted pressure section has a pore pressure gradient equal to that of a water column which permeability does not impede. Sections where the flow of pore fluids are restricted, by whatever mechanism, are called under-pressured, over-pressured, abnormally pressured, or geopressured.
An empirical relationship between seismic interval velocity and the pore pressure gradient is useful for predicting pore pressure gradients in areas where direct measurements are impractical (such as beneath the ocean floor). Seismic migration velocities are a precise measure of a specific average velocity type called Root Mean Squared (RMS) velocities. From RMS velocity, interval velocity, the average velocity over a specified interval, is calculated.
Experience has shown that the pore pressure gradient relates to interval velocity, and that the logarithm of the interval travel time (reciprocal of interval velocity as defined above) linearly relates to the logarithm of depth for the normally pressured regions of the subsurface. Over-pressured sections exhibit the same linear slope as normally pressured sections, but differ in intercept.
U.S. Pat. No. 5,343,440 to Kan et al. discloses a two-dimensional (2-D) geopressure analysis system. U.S. Pat. No. 5,128,866 to Weakley discloses a one-dimensional (1-D) pore pressure prediction method. However, neither Kan et al. nor Weakley disclosure a method for generating a 3-D pore pressure prediction or calibration field.
What is needed is a method which utilizes empirical relationships in order to predict the magnitude of the pore pressure gradient as a function of depth, latitude, and longitude.